(1) Field of the Invention
This invention relates to an organic electroluminescent element, and more particularly to such an element having improved durability, and a producing method thereof.
(2) Related Arts
Recently, in accordance with the diversification of information apparatuses, the demand for flat-type display elements has been growing which manage with less electric power and smaller space than cathode-ray tubes (CRT). Included in such flat-type display elements are crystalline liquid display elements and plasma display elements, and drawing special attention among them is an electroluminescent (hereinafter referred to as EL) element which is a self-luminescent type and can provide clear display.
Here, the EL elements can be divided between inorganic and organic depending on the constituent materials, the former having already been put to practical use.
However, such an inorganic EL element needs to be driven with high voltage because its driving type is a so-called `collisional excitation type` wherein electrons accelerated by the impression of a high electrical field make the luminescence center luminesce through the collisional excitation. This construction brings about an increase in the cost of surrounding devices. On the other hand, the organic EL element can be driven with low voltage because it exhibits so-called `injection type luminescence` wherein charges (holes injected from an electrode and electrons injected from another) are combined with each other inside luminous materials to produce luminescence. It has another advantage of being able to easily produce any desired luminous colors by altering the molecular structure of the organic compound. Hence, such organic EL elements are very hopeful as new display elements.
The organic EL elements generally have either two-layer or three-layer structure. The two-layer structure has either SH-A structure wherein a luminous layer and a hole transport layer are formed between a hole injection electrode and an electron injection electrode, or SH-B structure wherein a luminous layer and an electron transport layer are formed between these electrodes. The three-layer structure has DH structure wherein a luminous layer, a hole transport layer, and an electron transport layer are formed between these electrodes. Used for such hole injection electrodes are materials having a large work function such as gold and ITO (In-Sn oxide), while used for such electron injection electrodes are materials having a small work function such as Mg. All the layers comprise organic materials: the hole transport layer comprises a material having p-type semiconductor characteristics, the electron transport layer comprises a material having n-type semi-conductor characteristics, and the luminous layer comprises a material having n-type semi-conductor characteristics when used in the SH-A structure, a material having p-type semi-conductor characteristics when used in the SH-B structure, and a material having characteristics close to neutral when used in the DH structure. Every structure described above is based on the common principle that holes injected from a hole injection electrode and electrons injected from an electron injection electrode are combined on the boundary surface between a luminous layer and a hole (or electron) transport layer as well as inside the luminous layer to produce luminescence.
As mentioned before, the organic EL elements have advantages of being driven with low voltage, producing any desirable luminous colors theoretically, and the like. However, when it comes to the luminous durability, very few of them can retain stable luminescence. Thus, improving the durability has been an important subject.
The short lives of the present organic EL elements result from that the poor film-forming properties of the materials of the luminous layers and the electron transport layers cause undesired precipitates between adjacent layers laminated.
Another drawback of the organic EL elements is that even if both the electrodes are impressed low voltage to drive the elements, the electric fields among the elements are as high as 10.sup.5 V/cm, which may cause electron avalanche and then dielectric breakdown.
Further another drawback of the organic EL elements which comprise organic compounds is that their conductivity is low, so that the mobility of the carriers becomes low while the resistance becomes high. As a result, when voltage is impressed between the electrodes, the resistance of elements produces heat, which serves to melt or decompose the organic luminous layers, changing the luminous colors or hindering the emission.